1. Field of the Invention
The present invention relates to a light guide body having a prism formed surface longitudinally arrayed with a plurality of prisms and provided opposite to a light emitting surface, in order to allow light emitted from a light source to enter a light incident surface on a longitudinal end side and to emit from the longitudinally extending light emitting surface. The present invention also relates to a lighting apparatus and a document scanning apparatus that have the light guide body.
2. Description of Related Art
In a document scanning apparatus that scans an image on a scanned surface of a document, a light receiving element, such as a line CCD and the like, receives reflected light from a document surface illuminated with a lighting apparatus, and outputs an image signal. The lighting apparatus of the document scanner conventionally uses a fluorescent tube in general. In view of energy saving, however, a light source using an LED has recently become widespread.
Such a lighting apparatus using an LED as a light source has a configuration in which a light guide body extending along an entire width of a scanned area is used to guide light emitted from an LED light source to a document surface. In order to allow the light emitted from the light source to enter a light incident surface on a longitudinal end side and to be emitted from a longitudinally extending light emitting surface, the light guide body is provided with a prism formed surface on which a plurality of projecting prisms are arrayed in the longitudinal direction of the light guide body (refer to Related Art 1).
As a surface light source apparatus, a light guide body is disclosed having a substantially a triangular prism in a projecting shape on a surface opposite to a light incident surface (refer to Related Art 2).
In the lighting apparatus having such a configuration of the light guide body, commonly-called black floating may occur, in which an area of high brightness appears in a black-colored document surface. It is desired to prevent black floating in order to increase the quality of a scanned image.
FIG. 14 is a schematic view illustrating a state of light emission from a light guide body and light intensity in an optical reduction system in a conventional lighting apparatus. Light emitted from a light source 101 enters a light guide body 102. The light emitted from the light guide body 102 is radiated onto a document A. The reflected light from the document A is guided to a scanning sensor 104 through an optical reduction system 103. In the optical reduction system 103, the reflected light from the document A is reduced in a main scanning direction by a lens 105 so as to fit a width of a scanning sensor 104. Of the reflected light from the document A, only the light having a specific angle corresponding to the field angle of the lens 105 enters the scanning sensor 104.
In the light guide body 102, meanwhile, the light from the light source 101 entering the light guide body 102 and traveling toward the side opposite to the light source is reflected by a prism formed surface 106 and is emitted from a light emitting surface 107 (indicated with “a” in the drawing). Further, the light reflected by a mirror 108 provided to an end portion of the side opposite to the light source of the light guiding boy 102 and traveling in the light guide body 102 toward the light source side is reflected by the prism formed surface 106 and is emitted from the light emitting surface 107 (indicated with “b” in the drawing). The sum of the two lights emitted from the light guide body 102 has directivity.
In a region on the light source side in particular, the light emitted from the light guide body 102 has directivity in a direction inclined toward the side opposite to the light source as a main axis, relative to the direction orthogonal to the longitudinal direction (main scanning direction) of the light guide body 102. The intensity of the light in the main axis direction is the highest. Since the light in the main axis direction substantially coincides with the field angle of the lens 105, however, light receiving intensity is high on a light receiving surface of the scanning sensor 104. Meanwhile, toward the side opposite to the light source, the main axis direction of the emitted light shifts gradually to the direction orthogonal to the main scanning direction and the field angle of the lens 105 rapidly changes to a direction inclined toward the side on the light source. So, a difference between the main axis direction of the emitted light and the field angle of the lens 105 is greater toward the side opposite to the light source. Thus, the light receiving intensity on the light receiving surface of the scanning sensor 104 gradually decreases. The density in a scanned image then changes, thus causing black floating in which the scanned image looks pale on the light source side.
[Related Art 1] Japanese Patent Laid-Open Publication No. 2005-204329
[Related Art 2] Japanese Patent Laid-Open Publication No. H10-293304